As a vibration generating machine that can be used in a breaker machine, road roller machine and so forth, there has hitherto been known, for example, one that is disclosed in FIG. 1 of the drawings attached hereto.
More specifically, as shown in FIG. 1 it is seen that a unit body 1 has a piston 3 slidably inserted in a cylindrical bore 2 thereof. The said piston 3 is formed thereon with a large diameter portion 3a, a large diameter rod portion 3b and a small diameter rod portion 3c for defining with the correspondingly contoured surfaces of the said cylindrical bore 2 a first pressure receiving chamber 4 of a small pressure receiving area and a second pressure receiving chamber 5 of a large pressure receiving area, thus constituting a cylinder assembly 6. Further, a switching valve 9 is constructed by inserting a spool 8 slidably into a spool bore 7 of the said unit body 1 and defining a first pressure chamber 13 of a large diameter and a second pressure chamber 14 of a small diameter at both end sides of the said spool 8, respectively.
The above mentioned spool 8 is designed to establish and block a communication between a pump port 10, a principal port 11 and a tank port 12. Thus, the said spool 8 is thrusted to take a first position with a pressure fluid within the said first pressure chamber 13 of the large diameter to establish a communication between the said principal port 11 and the said tank port 12 and is thrusted to take a second position with a pressure fluid within the said second pressure chamber 14 of the small diameter to establish a communication between the said pump port 10 and the said principal port 11.
A servo valve 19 has a construction in which the above mentioned tank port 12 is normally allowed to communicate with a drain port 15 that is formed in the said cylindrical bore 2, the said first pressure chamber 13 is normally allowed to communicate with an auxiliary port 16 that is formed in the said cylindrical bore 2, and the said auxiliary port 16 is designed to establish and block, via a switching spool 17 that is integral with the said piston 3, a communication between the said drain port 15 and a first port 18. And, the said principal port 11 is allowed to communicate with a second port 20 whereas the said first port 18 and the said pump port 10 are supplied with a pressure fluid that is discharged from a hydraulic pump 21.
The above mentioned vibration generating apparatus is diagrammatically represented in FIG. 2 of the drawings attached hereto, in which the said first port 18 is shown as serving for both the said servo valve 19 and the said first pressure receiving chamber 4.
The above mentioned vibration generating apparatus will operate as set forth below.
When the said piston 3 is held at its neutral position as shown in FIGS. 1 and 2, the said spool 17 will act to block the said drain port 15, the said auxiliary port 16 and the said first port 18 so that the pressure fluid may be sealed within the said first pressure chamber 13. Then, the spool 8 (i.e. in the switching valve 9) will take a first position A to allow the said principal port 11 to communicate with the said tank port 12.
This will cause the said piston 3 to be moved in one direction (as shown by the arrow a) with the pressure fluid within the said first pressure receiving chamber 4. When it is so thrusted to take a position as shown in FIGS. 3 and 4, the said auxiliary port 16 will be allowed to communicate via a small diameter portion 17a of the said switching spool 17 with the said drain port 15 so that the pressure fluid may flow out of the said first pressure chamber 13 into a tank 22. The said spool 8 will then be thrusted to take a leftward position, i.e., a second position B as shown in FIG. 2, with the pressure within the said second pressure chamber 14 to establish a communication between the said pump port 10 and the said principal port 11.
This will cause the said second pressure receiving chamber 5 to be supplied with a pressure fluid and will thus cause the said piston 3 to be moved in the other direction (as shown by the arrow b) due to a difference in pressure based upon a difference in pressure receiving area.
When the said piston 3 is then thrusted to reach a position as shown in FIGS. 5 and 6, a communication between the said auxiliary port 16 and the said first port 18 will be established via the said large diameter rod portion 3b of the piston 3 to supply the said first pressure chamber 13 with a pressure fluid until the said spool 8 is thrusted to reach the said first position A shown in FIGS. 1 and 2 due to a difference in pressure based upon a difference in pressure receiving area so that the said piston 3 may be moved in the said one direction (shown by the arrow a). Such a series of operations will be repeated with the above mentioned vibration generating apparatus.
With such a vibration generating apparatus, it has been found, however, that when a said piston 3 is switched to move from a stroke end position in one direction into the other direction, a state will develop in which a pressure fluid is sealed in a said first pressure chamber 13. Thus, if the said piston 3 is moved slightly from a stroke end position in one direction as shown in FIGS. 1 and 2 into the other direction (as shown by the arrow b), a communication between a said auxiliary port 16 and a said drain port 15 will be blocked to bring about the the state in which the pressure fluid is sealed in the said first pressure chamber 13.
This will cause a pressure fluid in a said first pressure receiving chamber 4 to be leaked through a gap between a said cylindrical bore 2 and a said switching spool 19 to flow into the said first pressure chamber 13, and can, with a said spool 8 that may possibly be moved towards a said first position A, possibly result in a false operation.
More specifically, if the said spool 8 is allowed to reach the said first position A while the said piston 3 is being moved in the other direction, a said second pressure receiving chamber 5 will be caused to communicate with a said tank 22 so that the said piston 3 may be moved in the said one direction, i.e., reversely. Since this can result in a false operation, it may no longer be possible to ensure a regular reciprocating movement of the said piston 3.
With the above mentioned problems taken into account, it is, accordingly, an object of the present invention to provide a vibration generating apparatus which, without a piston suffering from a false operation, enables a reciprocating movement thereof to be carried out without fail.